This invention relates to a system for, and method of, determining atmospheric data relating to the movements of an airborne vehicle. More particularly, the invention relates to a system included in an airborne vehicle for using energy scattered from aerosol particles in the atmosphere to determine the vectorial speed of movement of the airborne vehicle relative to the aerosol particles. The system and method of this invention are especially adapted to determine the vehicle speed relative to that of the particles at a position sufficiently removed from the airborne vehicle to avoid any disturbance created by the movement of the airborne vehicle but sufficiently close to the airborne vehicle to indicate accurately the movement of the airborne vehicle with respect to the particles.
Mechanical instruments have long been used to measure the relative speed between a moving object such as an airborne vehicle and the free airstream through which the airborne vehicle is moving. The mechanical instruments determine the kinetic pressure exerted in a first defined area disposed on the vehicle in the direction of movement of the vehicle. The mechanical instruments also determine the static pressure exerted on a second defined area disposed on the airborne vehicle in substantially perpendicular relationship to the first defined area. The systems then compare the kinetic and static pressures to determine the relative air speed of the vehicle.
The mechanical instruments now in use typically employ Pitot tubes, pneumatic tubing and pressure transducers which are exposed to the external environment and are accordingly subject to degraded performance resulting from calibration changes from various causes such as component aging or changes in temperature. They are also subject to catastrophic failures as a result of accidental breakage. Furthermore, they protrude physically into the airflow.
As air navigation becomes increasingly complex, it becomes important to determine other data than the movement of the airborne vehicle relative to the ground. For example, it becomes increasingly important to know the characteristics of the air flow around the vehicle at each instant so that the response of the vehicle to such air flow can be properly controlled. For example, the air flow around the vehicle may affect the rate at which the yaw and pitch of the vehicles may be safely varied. The equipment now in use and discussed in the previous paragraphs has not been found satisfactory to provide the sensitive and accurate data which is now often required to control the rate at which the attitude of the vehicle can be safely varied.
A considerable effort has been made for a long period of time, and substantial sums of money have been expended during such period, to develop a system which will overcome the disadvantages discussed above. For example, systems have been developed using aerosol particles in the atmosphere to obtain desired air data. Such systems have directed energy from the airborne vehicle in such forms as substantially coherent light and/or radiation to the aerosol particles and have received coherent light scattered from the aerosol particles. Such systems have then processed the received signals to obtain the desired data. Although such systems appear to be promising, they have not yet demonstrated the performance that will be realized by this invention and they do not provide as accurate, sensitive and reliable information as may otherwise be desired and that will be attained by this invention.
In U.S. Pat. No. 4,887,213 issued to Anthony E. Smart and Roger P. Woodward on Dec. 12, 1989, for a "System For, and Methods of, Providing for a Determination of the Movement of an Airborne Vehicle in the Atmosphere" and assigned of record to the assignee of record of this application, a system is disclosed and claimed for overcoming the above disadvantages. In one embodiment, light generated from a moving airborne vehicle and scattered from particles in the atmosphere produces, at first and second detectors at the vehicle, signals indicative of such scattered light. The detected signals are converted in the system of U.S. Pat. No. 4,887,213 to digital signals. The digital signals from each particle are grouped. A centroid, based upon a weighting of the signals in each group in accordance with amplitude and time, is determined to represent the most probable time at which the particle crosses the peak of the illuminated region.
The peak amplitude of each signal from a first detector in the system of U.S. Pat. No. 4,887,213 is paired with the peak amplitude of the successive signals from a second detector. The time difference between the paired signals, and their product amplitudes, are determined. The amplitude products are separated into successive bins on the basis of the time difference between the signals in each pair. The amplitude products in each bin are averaged. The bin with the greatest average amplitude product and the two (2) adjacent time bins are then selected.
The median time in the bin in the system of U.S. Pat. No. 4,887,213 having the highest average product amplitude is used as a first approximation to the transit time of a particle between the two sheets. An estimate with enhanced accuracy may be obtained by calculating the "centroid", by a method analogous to that used above, of the distribution of events in the three (3) chosen bins. The movement of the airborne vehicle may be determined from the selected time difference.